Some of conventional television sets, video tape recorders or the like use a three-dimensional Y/C separating circuit adapted to process motions in pictures in order to improve separability of luminance signals and color signals. The three-dimensional Y/C separating circuit performs a still image Y/C separation processing for separating luminance signals and color signals by performing an arithmetic operation between screens, for example, between frames, and a moving image Y/C separation processing for separating luminance signals and color signals by performing an arithmetic operation in a screen, for example, between lines. The still image Y/C separation process based on the arithmetic operation between frames is also referred to as frame correlation separation processing, and the moving image Y/C separation processing based on the arithmetic operation between lines is also referred to as line correlation separation processing.
A three-dimensional Y/C separating circuit is described in detail in Japanese Patent Application Laid-Open Publication No. 2000-312366.
The three-dimensional Y/C separating circuit detects a motion in a picture based on the degree of the frame correlation, which is represented by the difference in input composite video signal between frames, and adaptively switches between the moving image Y/C separation processing and the still image Y/C separation processing based on the result of the motion detection. Therefore, in the three-dimensional Y/C separating circuit, motion detection and motion adaptive operation are important factors that determine the Y/C separation performance.
In the still image Y/C separation processing by the three-dimensional Y/C separating circuit, for example, since the phase of color signals of NTSC-format video signals is inverted every frame, luminance signals and color signals are separated by addition or subtraction of video signals between frames. In the moving image Y/C separation processing by the three-dimensional Y/C separating circuit, since the phase of color signals is inverted every line, luminance signals and color signals are separated by addition or subtraction of video signals between lines.
Both the Y/C separation processing based on the frame correlation and the Y/C separation processing the line correlation can separate luminance signals and color signals with reliability if the video signals to be subjected to the arithmetic operation are based on the same picture. In the case of a still image, the picture does not change between adjacent frames, so that the separation performance of the still image Y/C separation processing is high. In the case of a moving image, if video signals for vertically adjacent lines are highly correlated to each other, such as in the case of a relatively large pattern, the separation performance of the moving image Y/C separation processing is high. However, if video signals for vertically adjacent lines are poorly correlated to each other, such as in the case of a fine pattern, the separation performance of the moving image Y/C separation processing is relatively low. Therefore, the three-dimensional Y/C separating circuit determines whether a region that is the target of Y/C separation is a still image or a moving image so that the still image Y/C separation processing is performed when there is a frame correlation or, in other words, in the case of a still image, and sets a motion adaptive operation based on the result of the determination.
In motion determination, that is, frame correlation detection in the three-dimensional Y/C separating circuit, a 1-frame correlation detection for detecting a luminance signal motion and a 2-frame correlation detection for detecting a color signal motion are performed. Motion determination is carried out using the result of the 1-frame correlation detection and the 2-frame correlation detection.
A motion in an image can be determined by determining the difference between video signals for adjacent frames. However, color signals for adjacent frames are in opposite phase to each other, so that the difference between video signals for adjacent frames includes a color component. Thus, taking advantage of the fact that the color signal in the NTSC system lies in a band of 3.6 MHz±1.5 MHz, a low pass filter that allows signals at low frequencies (equal to or lower than 2 MHz, for example) to pass through is used to impose a band restriction on the difference between video signals for adjacent frames, so that motion detection for luminance signals can be achieved.
In this case, because of the band restriction, the 1-frame correlation is not sufficient to detect a fine motion in a picture and a motion in a color signal. Thus, a correlation between video signals for every two frames is detected. Color signals for every two frames are in phase with each other, and a motion in a color signal can be detected by determining the difference between video signals for every two frames.
However, while the Y/C separation processing uses video signals for adjacent frames to separate color signals and luminance signals, the difference between every two frames is determined for motion detection for color signals. Thus, in order to accurately detect a color signal motion component, a temporal filter is used. The temporal filter compares the magnitude of the current frame correlation and the magnitude of the frame correlation further delayed by one frame and detects the larger frame correlation. That is, the temporal filter selects the frame correlation for which the motion in the color signal is larger. However, the temporal filter poses a problem that the responsibility to motions in the input video signals becomes lower, because the temporal filter detects the motion in the color signal using not only the current frame correlation but also the frame correlation for the immediately preceding frame.
As described above, in the NTSC system, color signals for adjacent frames are in opposite phase to each other, and color signals for every two frames are in phase with each other. However, in the PAL system, color signals for every two frames are in opposite phase to each other, and color signals for every four frames are in phase with each other. That is, if such a three-dimensional Y/C separating circuit is applied to the PAL system, the difference between video signals for every two frames has to be determined in order to detect a motion in a luminance signal, and the difference between video signals for every four frames has to be determined in order to detect a motion in a color signal. As a result, in the case of application to the PAL system, there is a problem that a frame memory having a larger capacity than in the case of application to the NTSC system is required.
An object of the present invention is to provide a three-dimensional Y/C separating circuit that has a high responsibility to a motion in a video signal and can reduce a memory capacity.